PCP tuning question

[ER00z]

On a regulated pcp, are hammer spring settings and FPE output fairly linear at a certain regulator setting, given there's enough plenum volume to feed the valve regulated air?

I know this is a some what loaded question, and at some point increasing hammer strike (for more valve lift/duration) will gain little to nothing as you hit a plateau of the shot string (run out of plenum supplied air). But as you're on the down slope (very light hammer strike) to the "knee" of the regulated power curve, I was just curious if hammer spring adjustment gives fairly linear fpe output in half or whole turn (or whatever) increments.

[Ribbonstone]

You do have a good handle on the upper end of a regulated PCP and striker spring tension;  eventually there is just not any more useful air to give.

Don’t think a whole lot of people here have played with regulated PCP’s way-way down in the super low energy ranges.  Past a certain point, the velocity becomes erratic...which indicates the striker hit isn’t as constant at the low striker velocity a weak striker springs provide.

Not smart enough to figure out all the reasons for that...am reminded a physics class where we calculated the energy needed to get a dragged brick started….and how much it too to keep it moving aver it got started.  So I wrote it off to variable friction.

[ER00z]

Ribbonstone, thanks for the response.

I've been working my way through in attempt to learn the ways of regulated pcp's. I've only had hands on experience with one regulated pcp. Even after reading and digesting tons of information, in practice I've learned that "on paper" theories can get you close (usually 5-8% for me) but things don't always go as planned. Also, the variables from gun to gun vary greatly.

Luckily, I was able to get within ~3% (higher) of my target velocity after my last "on paper" theory. I did notice, each full turn on the hammer/striker spring adjustment yielded approximately the same FPE difference. Just was curious if this was "normal" or just luck with the particular gun in question. 

[TorqueMaster]

Ribbonstone, thanks for the response.

I've been working my way through in attempt to learn the ways of regulated pcp's. I've only had hands on experience with one regulated pcp. Even after reading and digesting tons of information, in practice I've learned that "on paper" theories can get you close (usually 5-8% for me) but things don't always go as planned. Also, the variables from gun to gun vary greatly.

Luckily, I was able to get within ~3% (higher) of my target velocity after my last "on paper" theory. I did notice, each full turn on the hammer/striker spring adjustment yielded approximately the same FPE difference. Just was curious if this was "normal" or just luck with the particular gun in question.

I'm new to PCPs too, and I'm saying you got lucky.  My situation is a .177 Avenger that in my opinion is way oversprung -- like it has the same spring that should be in a .22 or .25 version.  At pressures under 1800psi-ish, the hammer spring adjustment is essentially useless.  I suppose a nearly ZERO FPE change over the range of the adjustment is linear...but probably not what you meant...

So it depends on where the range of spring force adjustment falls relative to the "knee" of the pressure/velocity power curve.

If the spring is getting adjusted over a very small range of forces, it may look linear...

The second graph here shows velocity change over a range of spring adjustments.  It really matters, at your given pressure, what portion of the graph your spring allows you to cover.

https://hardairmagazine.com/ham-columns/tuning-regulated-pcp-airguns/

My gun's range of adjustment would be at the far left side -- equivalent to the "maximum to -1 turn" range.  Your range may be at the far right, where you get lots of velocity change over your full adjustment range.

Physics is not my strong suit -- anyone please shoot this down -- but my theoretical napkin doodles say airflow goes  as the spring force ^ 1.5  power.  If velocity is proportional to airflow (If I wasn't totally off base before, I should be now...), and muzzle energy goes as v^2, muzzle energy would go as spring force ^3.    Other physical factors will of course limit all this. 

[ER00z]

TorqueMaster (Bob), I can relate to your hammer spring adjustment woes. My regulated gun is a Hatsan BT65 .177, was way over sprung from the factory. To add insult to injury, all the porting is massive, great for making huge power, but does little for efficiency.

There are some differences between platforms, but I managed to collapse a few coils of the striker (hammer) spring and gain use of the adjuster. I actually went a little too far, as the hammer is in free flight at the absolute minimum preload setting, up to ~+4 turns. But, it's a 90gram hammer so free flight actually helped a lot. At zero preload and a 100-105bar (~1500psi) regulator setting, I'm making 15-16fpe with pellets (8.6-10gr) & 17fpe with 12.5gr slugs, then +4 turns yielded 17.5fpe, +5 turns 19fpe, +6 turns 21fpe (with slugs). On paper, with my plenum volume I figured it would peak out at about ~24fpe, but have yet to test. I'll agree I got very lucky, just sharing my setup. I never did take any physics classes, and am sure many are far above me, in regards to various equations, Lol.

I wish you the best in your tuning. I'm sure in time you'll have things sorted out.

[JimD]

I use enough hammer spring to empty the regulated chamber and often a little more since it seems to make my guns more accurate.  One of my guns, a Snow Peak P35, came set with a hammer spring lower than what would empty the regulated chamber.  This seemed to cause the regulator to not seal up real well and the first shot after the gun sat for awhile would be 15-20 fps higher in velocity.  Then the rest of the string would be fine.  Increasing the hammer spring to the point that velocity failed to increase seems to have solved that.  With low hammer spring settings I was getting an ES around 35 fps.  With it turned up to empty the chamber I get an ES of 10-15. 

Long way of saying the regulator may not function well with low hammer spring settings.  But if it does not affect your regulator, I guess you could use the hammer spring setting this way.  But I think the better thing to do is to use the regulator to change the velocity and use the hammer spring to let the air out and, in fine increments, to improve the accuracy.  Some people start their tuning with the hammer spring turned up all the way and decrease it until they see velocity decrease.  Then they fine tune.  I try turning it up to see if it increases.  If it does I keep going until it doesn't.  But however you get there, I think the right way to set up the gun is with the regulator and hammer spring balanced against each other.

[Ribbonstone]

Real low end….like 5-7 foot pounds, where no one else seems to test.  Just couldn't get good consistency with “sporters” type guts with higher output pressures and open porting. 

[Trucker3573]

I believe when you get the HS tension perfectly matched with the reg pressure you will see the most consistent shot to shot velocity. Have to play with the HS moving it in small increments and of course have a chronograph.

[ER00z]

Thanks for all the replies so far, and appreciate all the input.

To JimD and Trucker3573, you both state having the HS matched to the regulator pressure, seems very logical. Some day I'll have to get another regulated gun and learn more on tuning. The BT65 has given me the basics, but the mass of the hammer makes it a bit different from other guns, I think. I can run such a  minimum preload (free flight hammer) and have no issues from 1500psi to 2000psi making power/consistency. The trick for me was not making power, but at various settings the most I have gotten was between 10-20fps ES between shots. Mostly the first shot was faster. For a while I had to pause between shots for consistency, but the regulator has settled in (I guess you can say) and become very consistent recently. My last shot string had a 9fps spread, eight were within 4fps, which I think is very good, given the equipment I'm using.

Real low end….like 5-7 foot pounds, where no one else seems to test.  Just couldn't get good consistency with “sporters” type guts with higher output pressures and open porting. 

As low as I have gone with a pcp is 10ftlbs, but that one wasn't regulated. A project gun I'll be working on in the next year will be a .177 at 5-8ftlbs for a youth. Regulation hadn't even crossed my mind for that gun. I was just going to choke down the transfer port for a long, flat shot string. A low power "target gun" that's regulated seems like a very neat idea, but I think everyone chases big power and haven't seen much on lower power stuff. It's probably out there, I just haven't looked honestly. The lowest I've got with the BT65 was about 13ftlbs, with the 100bar reg setting, free flight hammer and an o-ring buffer on the valve stem between the striker plate and valve face, but consistency was awful...

[triggertreat]

Assuming ample plenum space of at least a 1/2cc per FPE for a desired tune, I would be focused on tuning just below the knee of a given reg set point for the best tune.  This would require finding the minimum HS adjustment needed or hammer energy needed to find the max velocity for the current set point and then backing off on the hammer energy by 3% to 5% of that max found to put you just below the knee which in turn will set the valve dwell for the best efficiency.  Tuning this way will give you the best shot count and the quietest shot report too given no hammer bounce is taking place.  If you didn't like the end result velocity, I would adjust the set point and start over.  I think you'll find leaning more towards a 5% drop from the max found and using a higher set point to reach your desired tune, which will require more hammer energy, will result in a shorter burst of air which will in turn give you a more consistent ES.

Once tuned for the best valve efficiency you should also be able to shoot past the set point up to a point for a number of more shots and still remain within the same ES that you were getting above the set point being you are tuned for the best valve efficiency.

If charting the string on a graph or watching the chronograph when shooting below the set point you should see a small bell cure taking place and then should see the ES begin to increase beyond this bell curve as the reservoir pressure drops too low to maintain a desired ES goal. 

[Ribbonstone]

Non regulated, but a real long sweet spot so experiments were valid.

A simple, but not so popular adjustment is the transfer port (if it has an easy to adjust restriction screw).  Seems to be more popular when it  has a “click knob” that has a few pre-sets...suxs when it’s hidden, hard to adjust, or prone to readjusting itself.

Get the fill pressure/striker force “right” and a AA 200T will go from and accurate 5-6 foot pounds to about 15-16 foot pounds.  Bump it up by 2 foot pounds or so for the old .22 AA 200S.

Know it’s out of favor mostly because it does little to change the total energy in vs energy out  efficiency equation.

[JimD]

It isn't regulated but I noticed increasing the transfer port size of my Prod increased the ES.  I felt it needed more power so I did it anyway.  But for a low energy target I think a smaller transfer port makes sense.  My Prod came with a .080, I increased it to .1110 if I remember right.  If I wanted sub 10 fpe I would try a .065.  Should give significantly more shots with a good ES. 

If I ever decide I want to shoot heavier pellets or slugs from my P35 I will probably mess with the transfer port - to make it bigger.  It's probably a bit small for a 25 caliber but I am just shooting 20 grain FTTs at about 875 fps so at this relatively low power level I think it's OK. 

To me a bigger transfer port gives you more power at a lower regulator setting but also tends to give you a bigger ES. 

[Mr.P]

Is there a rule of thumb for when to add or decrease hammer weight versus increasing or decreasing spring length/stiffness?  )For unregulated guns in this case)

[TorqueMaster]

TorqueMaster (Bob), I can relate to your hammer spring adjustment woes. My regulated gun is a Hatsan BT65 .177, was way over sprung from the factory. To add insult to injury, all the porting is massive, great for making huge power, but does little for efficiency.

There are some differences between platforms, but I managed to collapse a few coils of the striker (hammer) spring and gain use of the adjuster. I actually went a little too far, as the hammer is in free flight at the absolute minimum preload setting, up to ~+4 turns. But, it's a 90gram hammer so free flight actually helped a lot. At zero preload and a 100-105bar (~1500psi) regulator setting, I'm making 15-16fpe with pellets (8.6-10gr) & 17fpe with 12.5gr slugs, then +4 turns yielded 17.5fpe, +5 turns 19fpe, +6 turns 21fpe (with slugs). On paper, with my plenum volume I figured it would peak out at about ~24fpe, but have yet to test. I'll agree I got very lucky, just sharing my setup. I never did take any physics classes, and am sure many are far above me, in regards to various equations, Lol.

I wish you the best in your tuning. I'm sure in time you'll have things sorted out.

This is a very interesting thread, I'm glad you brought this up.  I've been tinkering with storebought springs that just happen to work for me, rather than modify the original.  I'm curious how you collapsed coils on the hammer spring?  Heat?  My crude attempts with pliers and such have failed or worse, so I just cut and grind flat.

Sounds like you've got it nailed down where you want it, that's what's important, physics may govern it, but grasping the basic concepts is enough for us mortals.

As others have said, and I'm just basing this off a lot of reading, mostly here, is getting the hammer/spring and valve balanced is key.  In the case of regulated, one needs to know where the reg will be set before one can do that effectively.  Velocity is most affected by reg setting, so knowing what your ammo wants largely determines pressure.  Choose pressure to get about 105% of desired velocity when "oversprung", so when "correctly" sprung, velocity should be where you want it and in the 3-5% below the plateau velocity.  On unregulated guns, they say the best, stablest ES is on peak and the down slope, where the hammer isn't just pecking at the valve, but opening it significantly -- so in a regulated gun the same should be true.  Creating that curve by running through different reg pressures, actually multiple curves, each on its own spring setting, helps show when the spring is too little, just right, or too much.  If I'm thinking correctly, a regulated gun should set up in the equivalent unregulated gun's "sweet zone", but being regulated, it's always in that sweet zone.  We get to choose which spot in the sweet zone.

@ JimD
That's interesting about hammer adjustment and regulator performance.  I think I saw that happening with a lighter/too light spring setting too -- pressure creeping up that last bit (50-100psi) over a few minutes, rather than just being there.  I just thought maybe my reg wasn't fully broken in yet, but I do not recall it ever happening with the stock, overpowered spring.  A good reason it may be better to be a bit towards oversprung rather than "just right" sprung or undersprung.

[triggertreat]

Opening the TP up more on an unregulated gun does become trickier when working with the ES.  As the pressure drops on the back of the poppet the velocity peak climbs higher at mid pressure as does the ES.  The trick is to go back in and rebalance the system again to settle the ES back down.

Some tips in doing this to save time and a lot of ammo is to just fill to mid pressure and record the peak velocity there and then fill the rest of the way up to note the starting pressure velocity which will then give you the full string ES without having to shoot a full string to get it while working to rebalance the system.

I find when opening up a TP that it is more productive to work with the valve spring first by lowering its spring rating more and then work with the hammer energy to fine tune more.  I typically custom cut different spring ratings for the hammer energy to fine tune and reach my ES goals.  Mine being within a 2% ES for a given string.  The goal is to get the full fill starting velocity to more closely match the new peak velocity in order to flatten the string more and thus lower the ES more.  You can cheat by lowering the fill pressure, but you'll lose shot count when doing so which is no fun.

[ER00z]

Is there a rule of thumb for when to add or decrease hammer weight versus increasing or decreasing spring length/stiffness?  )For unregulated guns in this case)

From my limited expirence with unregulated guns, I found easiest to get a chrony reading from a full fill all the way down to whare you want to refill from. Once you map out the velocity at various pressures you can go to adjusting the hammer spring tension.

If the velocity starts high and descends from high pressure to low, lower the preload of the spring (or possibly lower hammer weight, but that's more advanced "tuning") until velocity starts a little low then comes up, and then descends again (a bell curve).

If velocity starts very low then "spikes" at lower pressure (like the mid point or towards end of bell curve) then increase hammer spring preload (or hammer weight) to flatten the curve some.

Hammer strike force will determine the operating range of pressure. If you want a lower pressure fill, then you can get away with less hammer spring tension (generally).

Interesting thing is, as mentioned by others having a larger transfer port opening will give a higher velocity spread (a more pronounced bell curve). A larger TP will give more peak power, but if you're willing to trade more peak power for more consistency, you can reduce the TP diameter. The smaller the TP the less power, but more consistent.

When tuning your shot string, take the average FPE and multiply by the number of shots to get a rough estimate of total FPE (foot pounds of energy) per fill (air usage). So if you get 20 shots at 30fpe you get 600fpe total for the air usage. From there you can get an estimate of how many shots you'd get at various FPE output. So roughly 40 shots at ~15fpe or 10 shots at ~60FPE. Efficiency usually drops as you go higher in power, so shot count might not be as high, but efficiency will increase some as you go lower, so a few more shots could be had (possibly).

These have been my findings, they might be slightly off, Lol. But the basics to unregulated tuning   

[ER00z]

This is a very interesting thread, I'm glad you brought this up.  I've been tinkering with storebought springs that just happen to work for me, rather than modify the original.  I'm curious how you collapsed coils on the hammer spring?  Heat?  My crude attempts with pliers and such have failed or worse, so I just cut and grind flat.

TorqueMaster, yes I used heat and pliers. I used a mini torch to heat one spring bend at a time (red hot) then pinched flat with pliers. Probably best to do one coil at a time to test the effects. It might take a few attempts to collapse the coils so they're flat, and much easier to to do in lower light to see how much heat is going into the spring. How you hold the spring is the tricky part, I got away with a second set of pliers. Whatever used will act as a heat sink...

[ER00z]

Opening the TP up more on an unregulated gun does become trickier when working with the ES.  As the pressure drops on the back of the poppet the velocity peak climbs higher at mid pressure as does the ES.  The trick is to go back in and rebalance the system again to settle the ES back down.

Some tips in doing this to save time and a lot of ammo is to just fill to mid pressure and record the peak velocity there and then fill the rest of the way up to note the starting pressure velocity which will then give you the full string ES without having to shoot a full string to get it while working to rebalance the system.

I find when opening up a TP that it is more productive to work with the valve spring first by lowering its spring rating more and then work with the hammer energy to fine tune more.  I typically custom cut different spring ratings for the hammer energy to fine tune and reach my ES goals.  Mine being within a 2% ES for a given string.  The goal is to get the full fill starting velocity to more closely match the new peak velocity in order to flatten the string more and thus lower the ES more.  You can cheat by lowering the fill pressure, but you'll lose shot count when doing so which is no fun.

This is very good, I had to read over a few times to digest the information, lol.

The other thing to mention is the valve spring strength. Lowering the valve spring strength either by reducing the length or replacing with a weaker one, is equivalent to increasing hammer strike.

I found something very interesting regarding valve springs. There's a fellow who posted a video, who claims he removes the valve springs in (some of?) his pcp guns entirely. He relies just on the valve pressure to keep the (valve stem) poppet closed. Claims you only need the spring to get the pressure up during filling, but he uses a compressor (or bottle) to fill his guns. Going that route, he's getting away with very little hammer spring tension.

I'm not advising this, and am not certain it's a good idea for those who hand pump, but it's a very interesting concept.

[subscriber]

I understand adjusting the hammer spring tension to increase the hammer strike or reduce it, but where does the hammer stroke limiting adjustment fit in?  It seems to me that to reduce power, it may be beneficial to still open the valve with a fast moving hammer, to strike it hard enough to overcome the internal plenum pressure, but limit the valve travel and dwell by means of a shorter hammer stroke - set by the travel limiting screw.   

Not all PCPs have hammer travel limit adjustability.  Is this redundant with regulated PCPs?

[Ribbonstone]

Average PCP’s really only give you one (occasionally two) external tuning adjustments.  Most owners are happy to just adjust what is given.  Hobbyist will go inside and change things, or make parts that allow external adjustments.

(Are still PCP’s made without a single external adjustment.)
 
Generally, the external adjustments possible (but not all of them on any one PCP) are regulator pressure, striker spring tension, transfer port restriction, striker stroke.

One “freak” adjustment that Sumatra used was a variable collar around the valve stem...in function, it acted as an adjustable “bstaley o-ring mod”.

That’s not enough for experimenters, add in mods that  have to be done by replacing/changing parts. Porting changes, poppet shape/material changes, striker weight changes, adding air volume, etc.

Some of the adjustments act a little more independent from the others, but mostly they act in unison...change one, and others would be better if changed to suit.

The more adjustments, the more possible combinations.  Starts to approach the infinite.  With all we can think to do, we still eventually settle for the best found.

It’s often a search for “more” of something, with a counter balance of “less” or something else.   

More power (which decreases shot count) ...more shot count (which decreases power).   That pesky Ying always following right behind the Yang.